Field of the Invention
The invention concerns a method for controlling a magnetic resonance imaging system to generate image data of an examination subject, a control sequence determination system to determine such a control sequence, and a magnetic resonance imaging system designed for operation according to such a method.
Description of the Prior Art
Imaging systems that make use of a magnetic resonance measurement (signals originating from nuclear spins) are known as magnetic resonance tomography systems and have been successfully established and proven for a multitude of applications. In this type of image acquisition, a static basic magnetic field BO, which serves for initial alignment and homogenization of magnetic dipoles that are to be examined, is superimposed with a rapidly switched magnetic field (known as the gradient field) for spatial resolution of the imaging signal. To determine material properties of an examination subject to be imaged, the dephasing or relaxation time of the nuclear spins is determined after a deflection of their magnetization out of the initial alignment, such that different relaxation mechanisms or relaxation times, which are typical to the material, can be identified. The deflection most often takes place by radiating a number of RF pulses, and the spatial resolution is based on a chronologically established manipulation of the deflected magnetization with the use of the gradient field in a collection of pulses known as a measurement sequence, or control sequence, which establishes a precise chronological sequence of RF pulses, the change of the gradient field (by switching a sequence of gradient pulses) and the detection of measurement values.
An association between the measured magnetization, from which the noted material properties can be derived, and a spatial coordinate of the measured magnetization in the spatial domain in which the examination subject is situated, typically takes place with the use of an intermediate step. In this intermediate step, acquired raw magnetic resonance data are entered at readout points in a memory organized in an arrangement known as “k-space”, wherein the coordinates of k-space are coded as a function of the gradient field. The magnitude of the magnetization (in particular the transverse magnetization, defined in a plane transverse to the basic magnetic field) at a defined location of the examination subject can be determined from the data of the readout point, through a Fourier transformation that calculates the signal strength of the signal in the spatial domain from a signal strength (magnitude of the magnetization) that is associated with a defined frequency (the spatial frequency) or phase position.
The gradient field (in particular a characteristic thereof known as the gradient moment) defines a point in k-space, and the curve of the gradient field establishes a series of k-space points that can be designated as a “trajectory” through k-space, or also a “projection” in k-space.
Most often, k-space is scanned (filled by data entries made therein) as a series of readout points (known as sampling), with the distances between the readout points usually being predetermined according to the Nyquist-Shannon condition, and in addition are most often uniformly distributed in k-space. According to the Nyquist-Shannon sampling theorem, a sampling rate of k-space is predetermined for a defined, sought number of image points in each region of the subject (i.e. in the spatial domain) that is to be imaged (i.e. a desired spatial resolution of the image data). The minimum time between sampling events that results from adherence to a strict Nyquist-Shannon sampling rate can, in certain situations, be severely limiting, for example if the generation of image data of a moving examination subject is sought. For example, this can be the case for image data of a heart. A particularly fast acquisition of the magnetic resonance data can be required for a type of image presentation known as a CINE acquisition, for example if a “balanced steady state free precession” (bSSFP) magnetic resonance signals should be acquired as image information (raw magnetic resonance data that are acquired with a bSSFP method are designated in the following as “bSSFP raw data” for short). Often, a low spatial resolution is therefore selected in order to acquire bSSFP raw data.
Therefore, it would be desirable to be able to acquire image data more quickly with a predetermined quality (i.e. in particular with a predetermined spatial resolution), than with the minimum time determined via the Nyquist condition.